This invention relates to the field of non-linear laser optics generally, and specifically to the field of non-linear optical parametric mixing of laser beams of different frequency in non-linear mixing media to generate beams of new frequencies.
The traditional method for generating light at a frequency equal to the sum (.omega..sub.1 +.omega..sub.2) or the difference (.omega..sub.1 -.omega..sub.2) of the frequencies .omega..sub.1 and .omega..sub.2 of two available laser light sources is non-linear optical parametric mixing (OPM). Beams of frequencies .omega..sub.1 and .omega..sub.2 are combined in a material, usually a crystal, having non-linear susceptibility to produce an output beam having frequency equal to the sum or difference of the frequencies of the input beams. For input beams having a single longitudinal mode, the efficiency of OPM can be comparable to the efficiency for second harmonic generation in a non-linear mixing crystal, provided that the input beams are spatially and temporally well overlapped. This, however, is rarely the case. Usually, two or more longitudinal modes are present in at least one of the input beams, so that the beams are no longer temporally correlated and, as the result of intensity fluctuations in each of the beams (due to mode beating), output efficiency is substantially reduced.
It is an object of the present invention to improve the output efficiency of OPM sum or difference frequency generation from laser beams having two or more longitudinal modes.